1. Field of the Invention
The present invention relates to a liquid crystal on silicon panel and particularly to a liquid crystal on silicon panel which yields high image quality.
2. Description of the Prior Art
As an alternative to a conventional projector in which an image on a slide is projected onto a screen, a new-generation projector using a liquid crystal display element has been developed and is commercially available, referred to as an LCD projector. Generally speaking, there are three known projection type color image display methods for the liquid crystal display element. They are subtractive, temporal multiplexing, and spatial multiplexing display methods.
In the subtractive display method, three liquid crystal display elements corresponding to three primary colors are used. Optical systems for transmitting respective color lights of three primary colors and display elements for forming an image by controlling respective color lights are provided in pairs, independent of one another. Images of respective colors are optically superimposed on one another to display a full color image. Such a display device may have three individual light sources and color filters serving as source emitting the respective color beams of red, green and blue. Alternatively, the light emitted from the single white light source may be separated into color beams of three primary colors, i.e., red, green and blue, by dichroic mirrors.
In the temporal multiplexing display method, only one liquid crystal display element is used and color beams of three primary colors, i.e. red, green and blue are sequentially projected onto the liquid crystal display element. The liquid crystal display element forms images for the three primary colors sequentially and synchronously with the switching of the color beams. With a relatively high switching rate, the images of respective colors are visually superimposed on one another to display a full color image.
In the spatial multiplexing display method, the light is projected onto a liquid crystal display element having a color filter pattern of three primary colors in the form of a mosaic, by way of an optical system similar to a slide projector. The optical system can be simple in its construction and only one liquid crystal display element is used. Accordingly, the spatial multiplexing display method is suitable for a small projection type system.
FIG. 1 is a diagram showing a partial cross section of a conventional liquid crystal on silicon (LCoS) panel used in a spatial multiplexing display. It includes a silicon substrate 11, an Indium Tin Oxide (ITO) glass 12 used as a common electrode, a liquid crystal 15 sealed between the substrate 11 and the ITO glass 12, and color pixels composed of aluminum layers 131˜133 used as reflective pixel electrodes and color filters 141˜143 respectively of red, green and blue. An integrated driving circuit (not shown) disposed within the silicon substrate 11 generates voltage differences between the pixel electrodes 131˜133 and the common electrode 12. Each of the electric fields induced by the voltage differences twists the liquid crystal 15 within each color pixel region toward a desired direction, which forms a dark or bright or gray color pixel. The reflective pixel electrodes 131˜133 act as mirrors. When the light is projected onto the color pixels, it is colored and reflected onto a screen by the bright color pixels. Thus, using a liquid crystal display element having a matrix of rows and columns of these color pixels, an LCD projector generates a full color image on the screen.
In FIG. 1, it is noted that gaps 16 are located between the color pixels. The gaps 16 prevent the color filters 141˜143 from forming horn-shape portions. If the color filters 141˜143 are adjacent to one another without the gaps 16, the horn-shape portions easily form on the borders between the color filters 141˜143 due to misalignment in the manufacturing process.
However, the voltage differences between the common and pixel electrodes induce stray electric fields in the gaps 16 and the pixel edge areas M. The liquid crystal 15 located in the gaps 16 and the areas M is thereby twisted in unwanted directions. Such fringe effect degrades images in the pixel edge area, deteriorating image quality.